The invention relates to a lockup clutch hydraulic control system for fluid couplings having a lockup clutch, which enables slip control.
In the conventional practice, a lockup clutch is provided in the fluid coupling between the engine and transmission, such as a torque converter, for engaging and disengaging in accordance with automatic transmission curves for each gear, which directly connects the engine output to the transmission under certain conditions.
Consequently, if the current throttle setting and vehicle speed lie in the torque converter region of the automatic transmission curve, the lockup clutch disengages and the engine output is transmitted to the transmission via the torque converter. Conversely, if the current throttle setting and vehicle speed lie in the lockup clutch region of the automatic transmission curve, the lockup clutch engages and the engine output is transmitted directly to the transmission. However, when the lockup clutch is engaged, engine torque and speed fluctuations are directly transmitted to the transmission and propeller shaft, causing to vibration and noise problems.
A known solution to this problem is to permit torque transmission with a certain amount of slip of the lockup clutch, i.e., with the lockup clutch in a semi-engaged state. This system is shown in FIG. 9, where T is a torque converter; 201 is an impeller; 202 a turbine; 203 a stator; 204 a lockup clutch; 205 a cover shell; 206 a power input shaft; 207 a fixed shaft; 208 a one-way clutch; 209 a power output shaft; 210 a release chamber; 211 an apply chamber; 212 an oil-supply circuit; 213 an oil-discharge circuit; 215 an axial oil path; 216 an oil path; 217 a lockup control valve; 219, 220 rotational speed detectors; 221 a computer; 222 a pulse motor; 223 a rotating shaft; 224 a spool; 225, 226, 227 ports, 229 a spool bore; 230, 231 lands; 232 the regulator valve.
The torque converter T contains the impeller 201, the turbine 202, the stator 203 and the lockup clutch 204. The impeller 201 is welded to the cover shell 205 and the cover shell 205 is connected to the power input shaft 206. In addition, the stator 203 is supported through the one-way clutch 208 by the fixed shaft 207, which is fixed to the torque converter casing. The turbine 202 is splined to the output shaft 209, such that the two rotate together, and the lockup clutch 204 is provided between the turbine 202 and the cover shell 205. The interior of the torque converter T is divided into two chambers, the release chamber 210 and the opposing apply chamber 211 by the lockup clutch 204. Oil is supplied to the apply chamber 211 from the oil-supply circuit 212 and discharged through the oil-discharge circuit 213, so that oil pressure continuously acts on the torque converter T. The release chamber 210 is connected to the lockup control valve 217 via the axial oil path 215 in the output shaft 209 and the oil path 216.
Rotational speed detectors 219, 220, which are connected to the computer 221, are provided on the power input shaft 206 and power output shaft 209. The pulse motor 222 is also connected to the computer 221, and the male thread on the pulse motor rotating shaft 223 screws into the female thread on the spool 224 of the lockup control valve 217. The lockup control valve 217 comprises of spool bore 229 with three ports 225, 226, 227, and the spool 224 with two lands 230, 231. Port 225 is the drain port; 226 is connected to the release chamber 210 by the oil path 216; and port 227 is connected to the oil-supply circuit 212, which connects the apply chamber 211 to the regulator valve 232. The distance between the lands 230 and 231 on the spool 224, is formed to provide a small oil path between the ports 225 and 227.
The principle of operation of the system is as follows. At times when a high torque is required, such as when the vehicle starts moving, the computer 221 operates the pulse motor 222, in such a way that the spool 224 moves to the right, when viewed as shown in FIG. 9. This connects the ports 226 and 227 so that the same oil pressure is applied to both the release chamber 210 and apply chamber 211, and the lockup clutch 204 disengages.
During normal vehicle traveling, the 222 pulse motor keeps the spool 224 in approximately the position shown in FIG. 9. In this position, a small amount of oil is discharged from port 225 so that the oil pressure at port 226 is lower than that at port 227. As a result the oil pressure in the release chamber 210 is lower than that in the apply chamber 211, and the pressure difference between the two chambers pushes the lockup clutch 204 against the cover shell 205. During normal traveling, the signals from the rotational speed detectors 219, 220 are input into the computer 221, which calculates the speed ratio (power output shaft speed/power input shaft speed) between the power input shaft 206 and power output shaft 209. If this calculated ratio exceeds a prescribed value, the computer 221 sends a signal to the 222 pulse motor, which turns and forces the spool 224 a little to the right, reducing the amount of oil discharged from the port 225. Consequently, the pressure in the release chamber 210 rises and the slip between the lockup clutch 204 and the cover shell 205 increases, reducing the speed ratio. Conversely, if the speed ratio becomes too small, the spool 224 moves to the left, increasing the amount of oil discharged from port 225 and decreasing the slip between the lockup clutch 204 and the cover shell 205. Thus it is possible to maintain a prescribed speed ratio. If the vehicle speed exceeds a prescribed value, the spool 224 is forced to the left, connecting port 225 and port 226, discharging all oil from the release chamber 210, so that the lockup clutch 204 and the cover shell 205 are fully engaged.
This conventional approach uses a pulse motor to control the lockup control valve; but another method is also available, controlled by a solenoid, as shown in FIG. 10.
In this case, the lockup control valve 250 has ports 251, 252, 253, 255, 256, and the spool 257 has lands 259, 260, 261. Line pressure is supplied to ports 251, 253, 255 and the torque converter T apply chamber 262 via the regulator valve 263. Port 252 is connected to the release chamber 265 and port 256 is connected to the drain. The relationship of the external diameters D1, D2, D3, of the lands 259, 260, 261 of the spool 257 is: D2&gt;D3&gt;D1. In addition, the solenoid 266 is computer controlled, such that the needle valve 267 is forced out when the control signal is ON, closing off the line pressure drain circuit. Conversely, when the control signal is OFF, the needle valve 267 is withdrawn, opening the drain circuit. The ON-OFF control signal sent to the solenoid valve 266 is a 50 Hz signal, and by increasing the proportion of time that the signal is ON, the time the drain circuit is closed increases, so that the pressure at port 255 rises. Conversely, if the proportion of time that the signal is OFF is increased, the time the drain circuit is open increases and the pressure at port 255 does not rise.
This system operates as follows. In the torque converter region, the solenoid 266 is controlled so that the proportion of OFF time is increased and the amount of drain oil increases so that there is virtually no pressure rise at port 255 of the lockup control valve 250. Consequently, due to the difference in areas on which pressure acts of the lands 259, 260, 261 of spool 257, the spool 257 is forced to the right, closing the port 256 and connecting ports 252 and 253, and disengaging the lockup clutch 270. In this state, oil is discharged from oil-discharge circuit 268.
In the slip region (clutch semi-engaged) the computer controls the proportion of time that the solenoid 266 is ON such that the pressure at port 255 rises to a prescribed value. This prescribed pressure forces the spool 257 to the left so that land 260 opens port 256. When port 256 opens a small amount, the oil pressure between lands 252 and 253 is bled off and the spool 257 then moves back to the right. This process repeats, resulting in a high-frequency reciprocating oscillation of the spool 257 in the vicinity of port 256. This reciprocating oscillation intermittently opens and closes the oil-discharge circuits ports 252, 256 and the oil-supply circuits ports 252, 253, so that the pressure at port 252, ie. in the release chamber 265, is controlled to be lower than the pressure in the apply chamber 262, and the lockup clutch 270 slip is maintained at a constant level.
In the lockup region, the proportion of time that the solenoid 266 is ON is increased, decreasing the amount of oil drained from the solenoid 266, so that the pressure at port 255 of the lockup control valve 250 increases to a prescribed level. Spool 257 is forced to the left by this pressure, fully opening the path between the port 252 and the port 256, discharging the oil from the release chamber 265 and fully engaging the lockup clutch 270.
These two conventional approaches control the pressure difference across the lockup clutch to control the clutch slip by controlling the discharge pressure with a pulse motor or solenoid. Another control system exists, whereby the discharge pressure is connected to the drain and the oil supply pressure to the clutch is controlled.
In conventional systems, which employ a pulse motor or solenoid to control the discharge pressure, problems with unstable slip control occur because the relation between the solenoid pressure and the pressure difference acting across the lockup clutch valve is not constant unless the supply pressure to the lockup clutch is maintained constant. These systems have a problem that an oil pressure of 5-6 kg/cm.sup.2 must be supplied to the lockup clutch in order to transmit the maximum engine torque. Engine efficiency is reduced due to the waste in maintaining this high oil pressure under low-load conditions and detrimental structural effects are given on the torque converter.
In addition, in systems whereby the discharge is connected directly to the drain and the supply pressure on the lockup clutch is controlled, the discharge pressure is 0, when the torque is decreased so that the pressure applied to the clutch can drop too low if the torque decreases, leading to cavitation and pressure-instability problems.